Frequency multiplexing for qubit readout

ABSTRACT

A system includes a quantum processor includes a plurality of qubits. For each qubit, there is a circulator operative to receive a control signal and an output signal from the qubit. An isolator is coupled to an output of the circulator. A quantum-limited amplifier is coupled to an output of the isolator and configured to provide an output of the qubit. A multiplexor (MUX) is configured to frequency multiplex the outputs of at least two of the plurality of qubits as a single output of the quantum processor.

BACKGROUND Technical Field

The present disclosure generally relates to superconducting devices, andmore particularly, qubit readout.

Description of the Related Art

Superconducting quantum computing is an implementation of a quantumcomputer in superconducting electronic circuits. Quantum computationstudies the application of quantum phenomena for information processingand communication. Various models of quantum computation exist, and themost popular models include the concepts of qubits and quantum gates. Aqubit is a generalization of a bit that has two possible states, but canbe in a quantum superposition of both states. A quantum gate is ageneralization of a logic gate, however the quantum gate describes thetransformation that one or more qubits will experience after the gate isapplied on them, given their initial state.

SUMMARY

According to various embodiments, a method and system are provided forreading out signals from a quantum processor having a plurality ofqubits. In one embodiment, for each qubit, there is a filter coupled tothe qubit. A circulator is coupled to an output of the filter andoperative to receive a control signal. An isolator is coupled to anoutput of the circulator. A quantum-limited amplifier is coupled to anoutput of the isolator and configured to provide an output of the qubit.A multiplexor (MUX) is configured to frequency multiplex the outputs ofat least two of the plurality of qubits as a single output of thequantum processor.

In one embodiment, the control signal is at a different frequency foreach qubit.

In one embodiment, there is a readout resonator coupled between thequbit and the filter.

In one embodiment, the quantum-limited amplifier is a Josephsonparametric converter (JPC).

In one embodiment, the quantum-limited amplifier is a traveling waveparametric amplifier (TWPA).

In one embodiment, the quantum processor, the MUX, and the filter,circulator and quantum-limited amplifier of each qubit, are on a singleprinted circuit board.

In one embodiment, the quantum processor, the MUX, and the filter,circulator and quantum-limited amplifier of each qubit, are configuredin a dilution refrigerator.

In one embodiment, the MUX comprises one or more 90-degree hybridcouplers. Each 90-degree hybrid coupler is terminated with a resistor toground. The MUX may have multiple tapered stages configured tocollectively provide a single output for all outputs of the plurality ofqubits. Each 90-degree hybrid coupler may comprise transmission linesembedded in traces of a printed circuit board.

According to another embodiment, a method of reading out signals from aquantum processor comprising a plurality of qubits is provided. Thequantum processor receives a first input operative to measure a state ofa first qubit and a second input operative to measure a state of asecond qubit of the plurality of qubits. A first circulator routes thefirst signal to the first qubit. A second circulator routes the secondsignal to the second qubit. The first circulator routes an output of thefirst qubit in response to the first signal, to a first input of acoupler. The second circulator routes an output of the second qubit inresponse to the second signal, to a second input of the coupler. Theoutput of the first qubit and the output of the second qubit arecombined by frequency multiplexing, at a first output of the coupler.

In one embodiment, the coupler is a 90-degree hybrid. The second outputof the coupler may be terminated to ground via a resistor.

In one embodiment, the method is performed in a refrigerated environmentof a dilution refrigerator.

According to another embodiment, a system for combining the outputs of aplurality of qubits is provided. There is a coupler having two inputsand two outputs. A first circulator is coupled to a first qubit andconfigured to route a first signal to a first qubit and route an outputof the first qubit in response to the first signal, to a first input ofthe coupler. A second circulator is coupled to a second qubit andconfigured to route a second signal to a second qubit and route anoutput of the second qubit in response to the second signal, to a secondinput of the coupler. A first output of the coupler is operative toprovide an output that combines the output of the first qubit and theoutput of the second qubit by way of frequency multiplexing.

In one embodiment, the coupler is a 90-degree hybrid.

In one embodiment, a second output of the coupler is terminated toground via a resistor.

In one embodiment, the system is in a refrigerated environment of adilution refrigerator.

In one embodiment, the system further includes a third circulatorcoupled between the first circulator and the first input of the coupler.A fourth circulator is coupled between the second circulator and thesecond input of the coupler. A first amplifier coupled to the thirdcirculator and configured to amplify the output of the first qubitbefore it is provided to the first input of the coupler. A secondamplifier is coupled to the fourth circulator and configured to amplifythe output of the second qubit before it is provided to the second inputof the coupler. A third amplifier is at the first output of the coupler.

In one embodiment, the first and second amplifiers are Josephsonparametric converters (JPCs). The third amplifier is a wave parametricamplifier (TWPA).

These and other features will become apparent from the followingdetailed description of illustrative embodiments thereof, which is to beread in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings are of illustrative embodiments. They do not illustrate allembodiments. Other embodiments may be used in addition or instead.Details that may be apparent or unnecessary may be omitted to save spaceor for more effective illustration. Some embodiments may be practicedwith additional components or steps and/or without all the components orsteps that are illustrated. When the same numeral appears in differentdrawings, it refers to the same or like components or steps.

FIG. 1 illustrates an example architecture of a quantum computingsystem, consistent with an illustrative embodiment.

FIG. 2A illustrates a quantum computing system having a same number ofoutputs as inputs coming out of the dilution refrigerator for thequantum processor.

FIG. 2B illustrates a quantum computing system wherein the quantumprocessor has fewer outputs than inputs into the dilution refrigerator.

FIG. 3 is an example block diagram of a quantum computing system,consistent with an illustrative embodiment.

FIG. 4 illustrates a frequency multiplexing architecture, consistentwith an illustrative embodiment.

FIG. 5 illustrates a system of combining the outputs of two qubits,consistent with an illustrative embodiment.

FIG. 6 illustrates another system of combining the outputs of two qubitsinto a single output, consistent with an illustrative embodiment.

FIG. 7 presents an illustrative process related to reading out signalsfrom a quantum processor.

FIG. 8 is a functional block diagram illustration of a computer hardwareplatform that can be used to implement a particularly configuredcomputing device that can host the qubit control engine.

DETAILED DESCRIPTION Overview

In the following detailed description, numerous specific details are setforth by way of examples to provide a thorough understanding of therelevant teachings. However, it should be apparent that the presentteachings may be practiced without such details. In other instances,well-known methods, procedures, components, and/or circuitry have beendescribed at a relatively high-level, without detail, to avoidunnecessarily obscuring aspects of the present teachings.

The present disclosure generally relates to superconducting devices, andmore particularly, efficient readout of a quantum processor having aplurality of qubits. The electromagnetic energy associated with a qubitcan be stored in so-called Josephson junctions and in the capacitive andinductive elements that are used to form the qubit. In one example, toread out the qubit state, a microwave signal is applied to the microwavereadout cavity that couples to the qubit at the cavity frequency. Thetransmitted (or reflected) microwave signal goes through multiplethermal isolation stages and low-noise amplifiers that are used to blockor reduce the noise and improve the signal-to-noise ratio. Much of theprocess is performed in a cold environment (e.g., in a cryogenicchamber), while the microwave signal is typically measured at roomtemperature, but some lines of research perform the digitization insidethe fridge with either cryogenic CMOS or rapid single-flux quantumlogic). The amplitude and/or phase of the returned/output microwavesignal carries information about the qubit state, such as whether thequbit has dephased to the ground or excited state. The microwave signalcarrying the quantum information about the qubit state is usually weak(e.g., on the order of a few microwave photons). To measure this weaksignal with room temperature electronics (i.e., outside the refrigeratedenvironment), or even cryogenic electronics, low-noise quantum-limitedamplifiers (QLAs), such as Josephson amplifiers and travelling-waveparametric amplifiers (TWPAs), may be used as preamplifiers (i.e., firstamplification stage) at the output of the quantum system to boost thequantum signal, while adding the minimum amount of noise as dictated byquantum mechanics, in order to improve the signal to noise ratio of theoutput chain. In addition to Josephson amplifiers, certain Josephsonmicrowave components that use Josephson amplifiers or Josephson mixerssuch as Josephson circulators, Josephson isolators, and Josephson mixerscan be used in scalable quantum processors, as discussed in more detaillater.

The ability to include more qubits is salient to being able to realizethe potential of quantum computers. Reduction of the temperature of thecomputing environment below approximately 50 mK to as low as 2 mK isused for the quantum processor to function. Generally, performanceimproves as temperature is lowered, for example by reducing the residualthermally-excited state qubit population and decreasing the thermalbroadening of the qubit transition frequencies. Accordingly, the lowerthe temperature, the better for a quantum processor.

Applicants have recognized that to increase the computational power andreliability of a quantum computer, improvements are needed along twomain dimensions. First, is the qubit count itself. The more qubits in aquantum processor, the more states can in principle be manipulated andstored. Second is low error rates, which is relevant to manipulate qubitstates accurately and perform sequential operations that provideconsistent results and not merely unreliable data. Thus, to improvefault tolerance of a quantum computer, a large number of physical qubitsshould be used to store a logical quantum bit. For example, in this way,the local information is delocalized such that the quantum computer isless susceptible to local errors and the performance of measurements inthe qubits' eigenbasis, similar to parity checks of classical computers,thereby advancing to a more fault tolerant quantum bit.

Example Architecture

FIG. 1 illustrates an example architecture 100 of a quantum computingsystem, consistent with an illustrative embodiment. The architecture 100includes a quantum processor 112 comprising a plurality of qubits 114.The quantum processor 112 is located in a refrigeration unit 110, whichmay be a dilution refrigerator. A dilution refrigerator is a cryogenicdevice that provides continuous cooling to temperatures as low as 2 mK.Most of the physical volume of the architecture 100 is due to the largesize of the refrigeration unit 110. To reach the near-absolute zerotemperatures at which the system operates, the refrigeration unit 110may us liquid helium as a coolant. For example, a “dry” refrigerationunit may operate with two gaseous closed-cycle: one of He-4 that takesthe fridge down to 3K (the “pulse tube” cycle) and another of He-3/He-4mixture that takes the fridge down to 10 mK, or the lowest temperature(the “dilution” cycle). The only liquid in the system is inside thefridge, where the He-3/He-4 mixture condenses.

In the embodiment of FIG. 1, there is a measurement and control unit 130that is outside of the refrigeration unit 110. The measurement andcontrol unit 130 is able to communicate with the quantum processorthrough an opening 116, sometimes referred to as a bulkhead of thedilution refrigerator 110, that also forms a hermetic seal separatingthe ambient atmospheric pressure from the vacuum pressure of thecryostat under operation. A practical challenge in known refrigerationdevices that house qubits 114 is that the number of qubits that can beaccommodated in the refrigeration unit is limited due the number ofwires between the measurement and control unit 130 and the qubits 114measured thereby.

Consider for example a quantum processor 112 having 53 qubits 114. These53 qubits 114 may involve 53 input lines from the measurement andcontrol unit 130 (e.g., in order to control the qubits), andcorresponding 53 output lines (e.g., in order to measure the output ofthe qubits). There may be one amplifier per qubit, involving 53 “pumplines” to control the corresponding amplifiers of each qubit. Thus, 53qubits×3 lines per qubit=159 lines 120 between the refrigeration device110 and the measurement and control unit 130, as illustrated by the topview of the opening 122

As the number of qubits 114 increases, for example above 53 qubits tohundreds, thousands, or more, the opening 116 may not be large enough toaccommodate all the lines 120 supporting the quantum processor 112 inthe dilution refrigerator 110. Stated differently, access to the vacuumenvironment of the dilution refrigerator 110 is limited to the number ofconnectors that can fit through the bulkhead opening 116.

Accordingly, in one aspect, what is provided herein is an architecturethat substantially reduces the number lines between a measurement andcontrol unit 130 and a quantum processor 112 that is housed in arefrigerated environment. Multiple qubits 114 of the quantum processor112 can be read by way of frequency multiplexing the output signals ofthe qubits 114. These concepts are discussed in more detail below.

Example Block Diagrams

Reference now is made to FIG. 2A, which illustrates a quantum processor230 having a same number of outputs 240 coming out of the dilutionrefrigerator as those of inputs 220 leading to the quantum processor230. Qubit control and measurement involve individual addressability ofthe corresponding qubits, leading to many lines. These lines can beimplemented as radio frequency (RF) coax wires. For example, the inputlines may be operated at 5-7 GHz (for control and measurement) and theoutput lines 240 at around 7 GHz. As the number of qubits in the quantumprocessor 230 increase, the number of inputs 220 and outputs 240 growproportionally.

In contrast, FIG. 2B illustrates a quantum computing system 200B wherethe quantum processor 230 has fewer outputs 270 than inputs into thedilution refrigerator 210 for communication with the quantum processor230. Applicants have identified that once measurements are made of thecorresponding qubits in the quantum processor 230, they can bedifferentiated by frequency, thereby removing the need for separatelines (e.g., coax wires). The multiplexer 250, represented by way ofexample only and not by way of limitation as an 8:1 mux, allows thecombination of the measurements of each of the signals of thecorresponding qubits to be on one (or more) lines. For example, each ofthe outputs 260 from the quantum processor, is at a different frequency,which allows the readout of the plurality of qubits of the quantumprocessor 230 to be read out by way of frequency division multiplexing.For example, the readouts at the output 260 of the quantum processor 230are different for each qubit, such as, without limitation, 7.00 GHz,7.05 GHz, 7.10 GHz, 7.15 GHz, etc., to avoid cross-talk. In this way,the number of lines exiting the dilution refrigerator is substantiallyreduced, thereby allowing more qubits to be accommodated in a dilutionrefrigerator.

Reference now is made to FIG. 3, which is an example block diagram 300of a quantum computing system, consistent with an illustrativeembodiment. Quantum computing system 300 includes a quantum processorcomprising a plurality of qubits. As illustrated by way of example andnot by way of limitation by block 320(N), each qubit system 320(1) to320(N) may include a readout resonator 306 coupled to the qubit 302. Forexample, the readout resonator 306 affects a pulse coming from thecontrol/measurement instruments 316 at the readout resonator frequency(e.g., 7 GHz). The pulse acts as a measurement that decoheres the qubit302 and makes it collapse into a state of 1 or zero, thereby imparting aphase shift on that measurement pulse. The measurement of each readoutresonator of each qubit system 320(1) to 320(N) may be spread out infrequency, as mentioned previously (e.g., 7.00 GHz, 7.05 GHz, 7.10 GHz,7.15 GHz, etc.).

The readout resonator 306 may be coupled to a filter 310. Themeasurement fidelity is partially limited by the qubit 302 energyrelaxation through the resonator 306 into a transmission line, which isalso known as the Purcell effect. One way to suppress this energyrelaxation is to use the filter 310, which impedes microwave propagationat the qubit frequency. The circulator 314 routes the input from thecontrol/measurement instruments 316 to the filter 310 on to the qubit302, at which the reflected measurement in which the state of thedecohered qubit has been imparted, returns to the circulator 314 and isrouted to the input of an isolator 318 and on to the measurementinstruments (i.e. mixers, amplifiers, digitizers). This is an embodimentof a reflection measurement. Measurements may also be made intransmission, in which circulator 314 is not necessary, although thisidea makes the most sense for reflection.

In one embodiment, each qubit system 320(1) to 320(N) receives itscorresponding control signal from the control/measurement instrumentsblock 316, which lies outside the dilution refrigerator 350. In someembodiments, the control/measurement instruments 316 are inside thedilution refrigerator 350. There is an isolator 318 coupled to theoutput of the circulator 314. The isolator 318 is a two-port device thathas unidirectional transmission characteristics. Stated differently, theisolator 318 allows signals to propagate from the circulator 314 to theamplifier 320 but not from the amplifier 322 to the circulator 314. Anypower reflected from the load will be absorbed by the isolator 318, asopposed to being reflected back to the quantum limited amplifier 320.For example, isolator 318 can have approximately 40 dB of suppression inthe reverse direction. In various embodiments, different types ofisolators that can be integrated can be used, which include, withoutlimitation, a Multi-Path Interferometric Josephson ISolator (MPIJIS) andothers. It is noted that isolators may be built from circulators, andthe examples within, by terminating one of the three ports with anappropriate resistor (e.g., a 50 Ohm resistor). In various embodiments,the quantum limited amplifier 322 may be a Josephson parametricconverter (JPC) or a traveling wave parametric amplifier (TWPA). Forexample, JPCs are high-gain, low-noise amplifiers that can be used inthe readout of the qubit 302. JPCs are typically narrow band (e.g., 10MHz) but can be tuned over a wide range (e.g., 500 MHz) (i.e.instantaneous vs dynamical bandwidth). As to the TWPA, it uses anon-linear medium (e.g., transmission line that has resonators thatinclude Josephson junctions), or a non-linear medium such as a highkinetic inductance superconductor, where the inductance of the mediumdepends on the current that is put through it (i.e., thereby making itnon-linear). In one aspect, TWPAs discussed herein have a wide bandwidth(e.g., several GHz).

There is a frequency multiplexor (MUX) 330 that is operative to receivethe output signal of a plurality (or all) of the qubit systems 320(1) to320(N). The MUX 330 combines the signals and provides it to measurementinstruments 340, which are outside the dilution refrigerator 350. Themeasurement instruments 340 provide an output representative of a stateof each qubit from the corresponding qubit system 320(1) to 320(N).

In one embodiment, the qubit, readout resonator, filter, circulator,isolator, and amplifier of each qubit system, as well as the MUX, aremaintained at a low temperature within the dilution refrigerator 350.

Example Mux Implementation

Reference now is made to FIG. 4, which illustrates a frequencymultiplexing architecture 400, consistent with an illustrativeembodiment. The multiplexing circuit 402 includes one or more couplersthat are operative to couple the various signals received from qubits,represented by inputs ƒ₁ to ƒ_(N). In one embodiment, the couplers are90-degree hybrids. Other implementation of integrated couplers mayinclude, without limitation, Wilkinson power dividers, waveguidedirectional couplers (e.g., Bethe Hole coupler), Lange couplers, andothers. These couplers frequency multiplex the input signals ƒ₁ to ƒ_(N)in progressive stages (e.g., stage 1, stage 2, . . . stage N, with Neven in this embodiment) until the number of outputs is substantiallyreduced (e.g., to 1). For example, inputs ƒ₁ and ƒ₁ go through a90-degree hybrid, where a first output of the 90-degree hybrid providesan output ƒ₁+ƒ₁, whereas the second output is terminated (e.g., with a50-ohm resistance to ground). Similarly, input signals ƒ₃ and ƒ₄ arecombined in a first stage, whereas the second stage combines the sum ofƒ₁+ƒ₂+ƒ₃+ƒ₄. In the example of FIG. 4, the architecture 400 provides asingle output to outside the refrigerator. The signal can then beprocessed to determine the measured value of each corresponding qubit.For example, there may be a radio frequency (RF) power amplifier 430that receives the output signal and provides it to a mixer 432 (e.g.,single-sideband) that is coupled to a local oscillator. There may be anintermediate frequency amplifier (IF) 436 coupled to the output of themixer 432. There may be an analog to digital converter (ADC) 450 coupledto the output of the IF amp 436.

In one embodiment, each measurement signal ƒ₁ is unique. For example,the measurement signals ƒ₁ to ƒ_(N) may be at different frequencies,such as, without limitation, 7.00 GHz, 7.05 GHz, 7.10 GHz, 7.15 GHz,etc. Accordingly, once digitized by the ADC 450, the corresponding IFsignal is also unique and can be digitally discriminated by filters thatcan be implemented in software or hardware.

In various embodiments, the couplers of the multiplexing circuit 402 maybe integrated onto a chip or a circuit board, thereby reducing theoverall size of the architecture 400. By virtue of such integration,applicants have identified different structures that can be implemented.In this regard, reference is made to FIG. 5 which illustrates a system500 of combining the outputs of two qubits, consistent with anillustrative embodiment. For example, when performing a reflectionmeasurement of a qubit, an on-chip circulator, as depicted by way ofexample in FIG. 5 can be used.

An input ƒ₁ comes in to measure the state of the first qubit 502.Accordingly, the signal of the first input is routed by the firstcirculator 504 to the first qubit 502. The second input ƒ₁ is routed bythe second circulator 506 to the second qubit 508. The qubit dephasesand collapses and imparts a phase shift on the signal ƒ₁, which is thenrouted to the right (instead of left) by the circulator to the 90-degreehybrid 520. It is noted that circulators are normally 3-port devices,although they can be more, which take signals from ports 1→2, 2→3, 3→1(as indicated by the circular arrow, these ports do not need to belabelled), and are attenuated in the reverse direction, similar toisolators, although suppression is more like 25 dB in the reverse. Thismay be achieved by the nonlinear effect of Faraday rotation by themagnetic field of a ferrite material. The nonlinear effect is providedby different physical mechanisms known in the art. In variousembodiments, different types of circulators can be used, including,without limitation, mechanical on-chip microwave circulators,quantum-hall circulators, gyrators comprising Josephson mixers, andothers (all of which can also be used to realize isolators). Asubstantially similar operation occurs with respect to input signal ƒ₁.The circulator 506 ultimately routes the output of the second qubit 508to the right to the 90-degree hybrid coupler. The 90-degree hybrid thencombines the two measurement signals harvested from the first and secondqubits 502 and 508. In one embodiment, the entire operation is performedon one integrated chip. In one aspect, the coupling structure 500facilitates the measurement of qubits in reflection and to combine theresultant measurement signals inside a same package. In someembodiments, the same chip includes the terminator 530 (e.g., 50 Ohm, 75Ohm, or any other suitable impedance-matched termination resistance).

In some scenarios, the structure 500 may introduce losses, which can bemitigated by way of amplification within the integrated circuit. Forexample, the 90-degree hybrid coupler 520 may introduce a 3 dB loss. Inthis regard, quantum-limited amplifiers (e.g., JPC, TWPA) before orafter the hybrid can be used. Thus, even if there is loss, it can berecovered by way of local amplification at the integrated circuit level.In some embodiments, the terminator (e.g., 50 ohm) can be integratedonto the same package (e.g., via surface mount technology).

FIG. 6 illustrates another system 600 of combining the outputs of twoqubits into a single output, consistent with an illustrative embodiment.Some of the concepts embodied in system 500 are similar to those ofstructure 500 and are therefore not repeated here for brevity. System600 can be viewed as a more elaborate implementation of structure 500,which further includes amplification of each qubit output signal by wayof an amplifier (e.g., JPC) before the outputs of the qubit measurementsare combined by a coupler 620. More particularly, for the first qubit,602, a first signal ƒ₁ is applied to a first qubit 602 via a firstcirculator 604. A first output 605 is received from the first qubit 602,in response to the applied first signal ƒ₁. The first output 605 isrouted by a third circulator to the JPC amplifier, which is amplifiedthereby. JPC's are relatively narrowband (e.g., ˜10 MHz), and also workin reflection, which is why they particularly benefit from the thirdcirculator 609.

Similarly, for the second qubit 608, a second signal ƒ₁ is applied by asecond circulator 606. A second output 607 is received from the secondqubit 608 in response to the applied second signal ƒ₁. The second output607 is amplified by a second JPC amplifier 621. The amplified firstoutput is combined with the amplified second output by the coupler 620,which may be 90-degree hybrid. A first output of the 90-degree hybrid620 is amplified by an amplifier 620 (which may be a TWPA), wherein asecond output of the 90-degree hybrid 620 is terminated with atermination resistor 630 (e.g., 50 ohm).

Each of the systems 500 and 600 can be implemented on chip and/or on aprinted circuit board. By virtue of including the systems 500 and/or 600in one or more stages of a MUX, the number of wires that come out of thedilution refrigerator are substantially reduced.

Example Process

With the foregoing overview of the example architecture 100, conceptualblock diagram 200B, and frequency multiplexing architecture 400, it maybe helpful now to consider a high-level discussion of an exampleprocess. To that end, FIG. 7 presents an illustrative process related toreading out signals from a quantum processor. Processes 700 isillustrated as a collection of blocks, in a logical flowchart, whichrepresents a sequence of operations that can be implemented in hardware,software, or a combination thereof. In the context of software, theblocks represent computer-executable instructions that, when executed byone or more processors, perform the recited operations. Generally,computer-executable instructions may include routines, programs,objects, components, data structures, and the like that performfunctions or implement abstract data types. In each process, the orderin which the operations are described is not intended to be construed asa limitation, and any number of the described blocks can be combined inany order and/or performed in parallel to implement the process. Fordiscussion purposes, the process is described with reference to thearchitecture of system 500 of FIG. 5.

At block 702, the quantum processor receives a first input ƒ₁ operativeto measure a state of a first qubit 502. Similarly, at block 704, thequantum processor receives a second input ƒ₁ operative to measure astate of a second qubit 508.

At block 706, a first circulator routes the first signal ƒ₁ to the firstqubit 502. Similarly, at block 708, the second circulator 506 routes,the second signal ƒ₁ to the second qubit 508.

At block 710, the first circulator 504 routes an output of the firstqubit 502 in response to the first signal ƒ₁, to a first input of acoupler 520, which may be a 90-degree hybrid.

At block 712, the second circulator 506 routes an output of the secondqubit 508 in response to the second signal ƒ₁, to a second input of thecoupler 520.

At block 714, the coupler 520 combines the output of the first qubit 502and the output of the second qubit 506 by way of frequency multiplexing,at a first output of the coupler (i.e., ƒ₁+ƒ₂). The second output of thecoupler is terminated to ground via a termination resistor 530 (i.e., inthe embodiment of 90-degree hybrids). The process 700 is performed in adilution refrigerator. The quantum processor may have many qubits, whichare all implemented on an integrated circuit board or semiconductorchip. Each of the outputs of the qubits may be combined as describedherein by additional stages of couplers, collectively referred to hereinas the MUX, which may also be integrated onto the same circuit boardand/or chip, and operated within the dilution refrigerator. By virtue offrequency multiplexing the outputs of the qubits, the number of outputsof the quantum processor is substantially reduced.

Example Computer Platform

As discussed above, functions relating to determining the state ofvarious qubits of a quantum processor one or more computing devicesconnected for data communication via wireless or wired communication, asshown in FIG. 1, and in accordance with the process 700 of FIG. 7. FIG.8 provides a functional block diagram illustration of a computerhardware platform 600 that can be used to implement a particularlyconfigured computing device that can host a qubit control engine 840. Inparticular, FIG. 8 illustrates a network or host computer platform 800,as may be used to implement an appropriately configured computingdevice, such as the measurement and control block 130 of FIG. 1, as wellas the control instruments block 316 and measurement instruments block340 of FIG. 3.

The computer platform 800 may include a central processing unit (CPU)804, a hard disk drive (HDD) 806, random access memory (RAM) and/or readonly memory (ROM) 808, a keyboard 810, a mouse 812, a display 814, and acommunication interface 816, which are connected to a system bus 802.

In one embodiment, the HDD 806, has capabilities that include storing aprogram that can execute various processes, such as the qubit controlengine 840, in a manner described herein. The qubit control engine 840may have various modules configured to perform different functions. Forexample, there may be a control module 842 that is operative to sendcontrol signals to the qubit, similar to that of the control instrumentsblock 316 of FIG. 3. There may be a measurement module 844 operative toperform functions similar to that of the measurement instruments block340 of FIG. 3. There may be a frequency demultiplex block 848 that isconfigured to determine which signal corresponds to which qubit.

In one embodiment, a program, such as Apache™, can be stored foroperating the system as a Web server. In one embodiment, the HDD 806 canstore an executing application that includes one or more librarysoftware modules, such as those for the Java™ Runtime Environmentprogram for realizing a JVM (Java™ virtual machine).

Conclusion

The descriptions of the various embodiments of the present teachingshave been presented for purposes of illustration, but are not intendedto be exhaustive or limited to the embodiments disclosed. Manymodifications and variations will be apparent to those of ordinary skillin the art without departing from the scope and spirit of the describedembodiments. The terminology used herein was chosen to best explain theprinciples of the embodiments, the practical application or technicalimprovement over technologies found in the marketplace, or to enableothers of ordinary skill in the art to understand the embodimentsdisclosed herein.

While the foregoing has described what are considered to be the beststate and/or other examples, it is understood that various modificationsmay be made therein and that the subject matter disclosed herein may beimplemented in various forms and examples, and that the teachings may beapplied in numerous applications, only some of which have been describedherein. It is intended by the following claims to claim any and allapplications, modifications and variations that fall within the truescope of the present teachings.

The components, steps, features, objects, benefits and advantages thathave been discussed herein are merely illustrative. None of them, northe discussions relating to them, are intended to limit the scope ofprotection. While various advantages have been discussed herein, it willbe understood that not all embodiments necessarily include alladvantages. Unless otherwise stated, all measurements, values, ratings,positions, magnitudes, sizes, and other specifications that are setforth in this specification, including in the claims that follow, areapproximate, not exact. They are intended to have a reasonable rangethat is consistent with the functions to which they relate and with whatis customary in the art to which they pertain.

Numerous other embodiments are also contemplated. These includeembodiments that have fewer, additional, and/or different components,steps, features, objects, benefits and advantages. These also includeembodiments in which the components and/or steps are arranged and/orordered differently.

Aspects of the present disclosure are described herein with reference toa flowchart illustration and/or block diagram of a method, apparatus(systems), and computer program products according to embodiments of thepresent disclosure. It will be understood that each block of theflowchart illustrations and/or block diagrams, and combinations ofblocks in the flowchart illustrations and/or block diagrams, can beimplemented by computer readable program instructions.

These computer readable program instructions may be provided to aprocessor of an appropriately configured computer, special purposecomputer, or other programmable data processing apparatus to produce amachine, such that the instructions, which execute via the processor ofthe computer or other programmable data processing apparatus, createmeans for implementing the functions/acts specified in the flowchartand/or block diagram block or blocks. These computer readable programinstructions may also be stored in a computer readable storage mediumthat can direct a computer, a programmable data processing apparatus,and/or other devices to function in a manner, such that the computerreadable storage medium having instructions stored therein comprises anarticle of manufacture including instructions which implement aspects ofthe function/act specified in the flowchart and/or block diagram blockor blocks.

The computer readable program instructions may also be loaded onto acomputer, other programmable data processing apparatus, or other deviceto cause a series of operational steps to be performed on the computer,other programmable apparatus or other device to produce a computerimplemented process, such that the instructions which execute on thecomputer, other programmable apparatus, or other device implement thefunctions/acts specified in the flowchart and/or block diagram block orblocks.

The call-flow, flowchart, and block diagrams in the figures hereinillustrate the architecture, functionality, and operation of possibleimplementations of systems, methods, and computer program productsaccording to various embodiments of the present disclosure. In thisregard, each block in the flowchart or block diagrams may represent amodule, segment, or portion of instructions, which comprises one or moreexecutable instructions for implementing the specified logicalfunction(s). In some alternative implementations, the functions noted inthe blocks may occur out of the order noted in the Figures. For example,two blocks shown in succession may, in fact, be executed substantiallyconcurrently, or the blocks may sometimes be executed in the reverseorder, depending upon the functionality involved. It will also be notedthat each block of the block diagrams and/or flowchart illustration, andcombinations of blocks in the block diagrams and/or flowchartillustration, can be implemented by special purpose hardware-basedsystems that perform the specified functions or acts or carry outcombinations of special purpose hardware and computer instructions.

While the foregoing has been described in conjunction with exemplaryembodiments, it is understood that the term “exemplary” is merely meantas an example, rather than the best or optimal. Except as statedimmediately above, nothing that has been stated or illustrated isintended or should be interpreted to cause a dedication of anycomponent, step, feature, object, benefit, advantage, or equivalent tothe public, regardless of whether it is or is not recited in the claims.

It will be understood that the terms and expressions used herein havethe ordinary meaning as is accorded to such terms and expressions withrespect to their corresponding respective areas of inquiry and studyexcept where specific meanings have otherwise been set forth herein.Relational terms such as first and second and the like may be usedsolely to distinguish one entity or action from another withoutnecessarily requiring or implying any actual such relationship or orderbetween such entities or actions. The terms “comprises,” “comprising,”or any other variation thereof, are intended to cover a non-exclusiveinclusion, such that a process, method, article, or apparatus thatcomprises a list of elements does not include only those elements butmay include other elements not expressly listed or inherent to suchprocess, method, article, or apparatus. An element proceeded by “a” or“an” does not, without further constraints, preclude the existence ofadditional identical elements in the process, method, article, orapparatus that comprises the element.

The Abstract of the Disclosure is provided to allow the reader toquickly ascertain the nature of the technical disclosure. It issubmitted with the understanding that it will not be used to interpretor limit the scope or meaning of the claims. In addition, in theforegoing Detailed Description, it can be seen that various features aregrouped together in various embodiments for the purpose of streamliningthe disclosure. This method of disclosure is not to be interpreted asreflecting an intention that the claimed embodiments have more featuresthan are expressly recited in each claim. Rather, as the followingclaims reflect, inventive subject matter lies in less than all featuresof a single disclosed embodiment. Thus, the following claims are herebyincorporated into the Detailed Description, with each claim standing onits own as a separately claimed subject matter.

What is claimed is:
 1. A system, comprising: a quantum processorcomprising a plurality of qubits; for each of at least two of theplurality of qubits: a circulator operative to receive a control signaland an output signal from the qubit; an isolator coupled to an output ofthe circulator; and a quantum-limited amplifier coupled to an output ofthe isolator and configured to provide an output of the qubit; and amultiplexor (MUX) configured to frequency multiplex the outputs of theat least two of the plurality of qubits as a single output of thequantum processor, wherein the MUX comprises one or more 90-degreehybrid couplers.
 2. The system of claim 1, wherein the control signal isat a different frequency for each qubit.
 3. The system of claim 1,further comprising: a filter coupled to the qubit; and a readoutresonator coupled between the qubit and the filter.
 4. The system ofclaim 1, wherein the quantum-limited amplifier is a Josephson parametricconverter (JPC).
 5. The system of claim 1, wherein the quantum-limitedamplifier is a traveling wave parametric amplifier (TWPA).
 6. The systemof claim 1, wherein the quantum processor, the MUX, a filter, thecirculator and the quantum-limited amplifier of each qubit, are on asingle printed circuit board.
 7. The system of claim 1, wherein thequantum processor, the MUX, a filter, the circulator and thequantum-limited amplifier of each qubit, are configured in a dilutionrefrigerator.
 8. The system of claim 1, wherein each 90-degree hybridcoupler is terminated with a resistor to ground.
 9. The system of claim8, wherein the MUX has multiple tapered stages configured tocollectively provide a single output for all outputs of the plurality ofqubits.
 10. The system of claim 8, wherein each 90-degree hybrid couplercomprises transmission lines embedded in traces of a printed circuitboard.
 11. A system for combining the outputs of a plurality of qubits,comprising: a coupler having two inputs and two outputs; a firstcirculator coupled to a first qubit and configured to route a firstsignal to a first qubit and route an output of the first qubit inresponse to the first signal, to a first input of the coupler; and asecond circulator coupled to a second qubit and configured to route asecond signal to a second qubit and route an output of the second qubitin response to the second signal, to a second input of the coupler,wherein a first output of the coupler is operative to provide an outputthat combines the output of the first qubit and the output of the secondqubit by way of frequency multiplexing.
 12. The system of claim 11,wherein the coupler is a 90-degree hybrid.
 13. The system of claim 11,wherein a second output of the coupler is terminated to ground via aresistor.
 14. The system of claim 11, wherein the system is in arefrigerated environment of a dilution refrigerator.
 15. The system ofclaim 11, further comprising: a third circulator coupled between thefirst circulator and the first input of the coupler; a fourth circulatorcoupled between the second circulator and the second input of thecoupler; a first amplifier coupled to the third circulator andconfigured to amplify the output of the first qubit before it isprovided to the first input of the coupler; a second amplifier coupledto the fourth circulator and configured to amplify the output of thesecond qubit before it is provided to the second input of the coupler;and a third amplifier at the first output of the coupler.
 16. The systemof claim 11, wherein: the first and second amplifiers are Josephsonparametric converters (JPCs); and the third amplifier is a waveparametric amplifier (TWPA).